1. Field of the Invention
The present invention relates to a method of calculating a phase difference (offset) between a rotor reference position of an electric motor and an origin of a rotational position-detecting device resulted from, e.g., the fact that the rotational position-detecting device mounted on the electric motor is located out of position, and to a motor control device using such calculation method.
2. Description of the Related Art
There is a conventional motor control device that supplies (in both directions) an electric power from a power supply (hereinafter, referred to as inverter) capable of controlling a frequency and voltage, current, and phase to an AC motor (hereinafter, described as a synchronous motor here) thereby controlling a torque or a rotational speed thereof.
In this conventional motor control device, it is necessary to control phases of, e.g., voltage to be supplied based on rotational angles of an electric motor, so that a rotational position detecting device capable of outputting a rotational angle of a rotor of the electric motor as an instantaneous data is mounted on the electric motor. However, many rotational position detecting devices are mounted somewhat out of a real origin position of a rotor, so that it is necessary to compensate rotational angle signals having been detected, and to use them.
Although a variety of compensation methods have been proposed, what is important is that a phase difference between a rotor reference position and an origin of the rotational position detecting device is calculated and compensated. In this sense, for example, the Japanese Patent Application No. 054472/2003 has proposed a method.
The principle of calculation of a phase difference between a rotor reference position of an electric motor and an origin of a rotational position detecting device resulted from, e.g., the fact that the rotational position detecting device, which is mounted on the electric motor, is located out of position as is disclosed in the Japanese Patent Application No. 054472/2003, is now described on the supposition that the motor is a three-phase synchronous motor.
Letting a direction of the magnetic flux of a rotor d-axis, and a direction orthogonal to a magnetic flux of the rotor q-axis (it is also referred to as a control axis), voltage equations of d-axis and q-axis components, in the case where a phase difference between a rotor reference position and an origin of a rotational position detecting device is zero, will be expressed as follows.Vd=Rid−ωφq  (1)Vq=Riq+ωφd  (2):where Vd is a d-axis voltage, Vq is a q-axis voltage, and R is a resistance of one phase; id is a d-axis current, iq is a q-axis current, and φd is a d-axis component magnetic flux; and φq is a q-axis component magnetic flux, and ω is an angular velocity of a rotor.
When id=o and iq=0 in the above equation, φd and φq will be expressed as follows:φd=Ldid+φf=φf  (3)φq=Lqiq=0  (4):where Ld is a d-axis inductance, Lq is a q-axis inductance, and φf is a magnetic flux of a rotor.
Voltage equations of d-axis, q-axis components at this time will be expressed as follows:Vd=0  (5)Vq=ωφf  (6)Thus, Vd is zero.
However, in the case where there is a phase difference η between a rotor reference position and a detection output from a rotational position detecting device, currents id, iq of an electric motor will be transformed to id′, iq′ will be expressed as follows:id′=id cos η−iq sin η  (7)iq′=id sin η+iq cos η  (8)
Voltage equations of d′-axis, q′-axis components at this time will be expressed as follows:Vd′=Rid′−ωφq cos η+ωφd sin η  (9)Vd′=Riq′+ωφq sin η+ωφd cos η  (10)
At this time, even if id′=0, and iq′=0,Vd′=ωφf sin η  (11)Vq′=ωφf cos η(12)Thus, Vd′ is not zero.
At this time, it becomes necessary to calculate such an origin offset that Vd is zero. As for such calculation method, there are the followings:
(1) The method of sequentially adding an arithmetical progression (1°, 2°, . . . n°) to an output from the above-mentioned rotational position detecting device as a phase compensation amount when a d-axis voltage command value is not zero, and continuing the addition until a d-axis voltage command value comes to be zero.
(2) The method of calculating an arc tangent of a d-axis voltage command value and a q-axis voltage command value, and adding this arc tangent to an output from the above-mentioned rotational position-detecting device as an origin offset.
(3) The method of scanning at regular intervals of a predetermined angle from 1° to 180° as phase compensation amounts to record d-axis voltage command values when the above-mentioned d-axis voltage command value is not zero, and then making the interpolation from two phase compensation amounts the above-mentioned d-axis voltage command values of which are close to zero to calculate an origin offset.
However, in the case of intending to calculate an origin offset from a magnetic flux component voltage command value according to the above-described conventional art, since it is the determination based on instantaneous values, current values of the above-mentioned magnetic flux component and a component orthogonal to this component are not constant due to influence of, e.g., higher harmonic components of voltage or noise. Accordingly, there are some cases where current values of a magnetic flux component and a component orthogonal to this component are not zero even if voltage command values of a magnetic flux component are zero. Thus, a problem exists in the decrease of calculation accuracy of an origin offset.
Furthermore, in the case where there is any detection error in a current detector, the detection error is outputted even if an actual current value is zero. Thus, values obtained by the coordinate transformation of these detection outputs are not constant values, but values that periodically fluctuate. Even if any control is made to cause these values to be zero, voltage command values are not zero but values that periodically fluctuate. Thus, a problem exists in the decrease of calculation accuracy of an origin offset.
Moreover, current values or voltage command values after the coordinate transformation fluctuate due to the change in rotation. Thus, a problem exists in the decrease of calculation accuracy of an origin offset.
In other words, a problem exists in that high calculation accuracy cannot be obtained by the conventional calculation method of an origin offset.